1. Technical Field
This invention relates to power supplies and has particular relation to power supplies which can supply power efficiently at light loads and low voltages and with low noise at heavy loads.
2. Background Art
Transceivers, especially for wireless telephones, have a number of requirements. The telephone is usually battery powered, which means that the power supply must be efficient in order to extend battery life between recharges. The telephone is usually portable, which means that it (and therefore its power supply) must be both compact and light. Finally, the (mobile) telephone is in communication with a (fixed) base station, which means that the distance between the telephone and the base station will vary widely. This in turn means that the power required for sending and receiving signals will also vary widely.
The Switch Mode Power Supply (SMPS) is a DC-to-DC power supply. The SMPS design makes it very efficient, and thus makes it widely used in wireless telephones and other battery-powered electronic devices. As the name implies, it rapidly switches a DC voltage on and off, but with a well-controlled duty cycle, thus allowing the output voltage to be similarly well controlled. This good control extends over a broad range of battery conditions and an even broader range of power demands. The size of the SMPS is generally selected to be as small (compact, light, and cheap) as possible while still assuring that the maximum designed power requirements of the telephone (or other load) will be met.
Applicant has noted a design opportunity which has been overlooked by the prior art: when a load has a requirement for varying amounts of power, a low voltage is beneficial during low power conditions, and low noise is required during high power conditions.
When the power supply load power requirement is low a low voltage is beneficial if, for instance, the current drawn by the load is largely insensitive to the applied voltage. This is true when the load is a typical radio frequency power amplifier in a wireless telephone. In this case a lower voltage reduces the power drawn from the battery.
Using again the example of a mobile telephone power amplifier, it is important that the power supply has low noise and high voltage when the amplifier presents a heavy load. In this case the transmitted power is high, and (in advanced systems, such as cdmaOne or cdma2000), the received power is low. When the received power is low it is vital that the power supply does not introduce excessive noise into the system that could mask the received signal. The high voltage allows the high transmitted power to be more easily generated.
To achieve these joint goals (low noise and high voltage at high power and low voltage at low power) the applicant employs a SMPS at low power and bypasses the SMPS at high power. This has a number of advantages. Since, for high power demands, power flows from the battery to the power amplifier without passing through the SMPS, no noise from the SMPS is introduced into the power amplifier. Because the SMPS does not have to handle this top portion of the load""s power requirements, it can be made more compact, lighter, and cheaper. Because it is smaller, the SMPS consumes lower power itself, even when it is switched into the circuitxe2x80x94and, of course, consumes no power at all when it is switched out of the circuit. Most importantly, power consumption by the power amplifier is reduced. When the transition between the SMPS and bypass is properly arranged the power amplifier can spend the majority of time operating from an economical low voltage. This is especially true when operating within a system with advanced power control, such as cdmaOne or cdma2000.